Patch sent upstream to add "/dev/mapper" to the list of allowed paths for the CheckPool function.  Even though /dev/mpath isn't allowed, it was at one time and was left for legacy, see:

http://www.redhat.com/archives/libvir-list/2015-June/msg01231.html

Patches pushed upstream:

commit dbad00189958db304295519842b28342a005a3bc
Author: John Ferlan <jferlan>
Date:   Wed Jun 24 07:46:47 2015 -0400

    mpath: Update path in CheckPool function
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1230664
    
    Per the devmapper docs, use "/dev/mapper" or "/dev/dm-n" in order to
    determine if a device is under control of DM Multipath.
    
    So add "/dev/mapper" to the virFileExists, leaving the "/dev/mpath"
    as a "legacy" option since it appears for a while it was the preferred
    mechanism, but is no longer maintained
$ git describe dbad00189958db304295519842b28342a005a3bc
v1.2.17-rc1-12-gdbad001
$

PASSED
verified on:
libvirt-1.2.17-2.el7.x86_64
qemu-kvm-rhev-2.3.0-9.el7.x86_64
kernel-3.10.0-290.el7.x86_64





the virStorageBackendMpathCheckPool (checkPool) is called by storagePoolUpdateState (src/storage/storage_driver.c), which as follow

static void
storagePoolUpdateState(virStoragePoolObjPtr pool)
{
    bool active;
    virStorageBackendPtr backend;
    int ret = -1;
    char *stateFile;

    if (!(stateFile = virFileBuildPath(driver->stateDir,
                                       pool->def->name, ".xml")))
        goto error;

    if ((backend = virStorageBackendForType(pool->def->type)) == NULL) {
        VIR_ERROR(_("Missing backend %d"), pool->def->type);
        goto error;
    }

    /* Backends which do not support 'checkPool' are considered
     * inactive by default.
     */
    active = false;
    if (backend->checkPool &&
        backend->checkPool(pool, &active) < 0) {     // so if mpath and mapper dirs are not existing, "active" will be set to false
        virErrorPtr err = virGetLastError();
        VIR_ERROR(_("Failed to initialize storage pool '%s': %s"),
                  pool->def->name, err ? err->message :
                  _("no error message found"));
        goto error;
    }

    /* We can pass NULL as connection, most backends do not use
     * it anyway, but if they do and fail, we want to log error and
     * continue with other pools.
     */
    if (active) {  // when mpath and mapper dirs are not existing, this segment will be ignored.
        virStoragePoolObjClearVols(pool);
        if (backend->refreshPool(NULL, pool) < 0) {
            virErrorPtr err = virGetLastError();
            if (backend->stopPool)
                backend->stopPool(NULL, pool);
            VIR_ERROR(_("Failed to restart storage pool '%s': %s"),
                      pool->def->name, err ? err->message :
                      _("no error message found"));
            goto error;
        }
    }

    pool->active = active;   // when mpath and mapper dirs are not existing, pool will be set as inactive in libvirt.
    ret = 0;
 error:
    if (ret < 0) {
        if (stateFile)
            unlink(stateFile);
    }
    VIR_FREE(stateFile);

    return;
}


So my test scenarios are as follow:==================
1. Make sure neither of /dev/mapper and /dev/mpath are existing, define and start a mpath pool, restart libvirtd and check the mpath pool status. ("inactive" expected)
2. Make sure /dev/mapper dir exists, define and start a mpath pool, restart libvirtd and check the mpath pool status. ("active" expected)
3. Make sure /dev/mpath dir exists, define and start a mpath pool, restart libvirtd and check the mpath pool status. ("active" expected)


Prepare a mpath pool=============================
#cat mpath.pool
<pool type='mpath'>
  <name>mpath</name>
  <available unit='bytes'>0</available>
  <source>
  </source>
  <target>
    <path>/dev/mapper</path>
  </target>
</pool>

# virsh pool-define mpath.pool
Pool mpath defined from mpath.pool

# virsh pool-start mpath
Pool mpath started

# virsh pool-list | grep mpath
 mpath                active     no 


Scenario 1:======================================
1. # service multipathd stop
Redirecting to /bin/systemctl stop  multipathd.service
2.# rm -fr /dev/mpath/; rm -fr /dev/mpath/

3. # service libvirtd restart
Redirecting to /bin/systemctl restart  libvirtd.service
4. # virsh pool-list --all
 Name                 State      Autostart
-------------------------------------------
...   
 mpath                inactive   no        <=== inactive as expected.
...


Scenario 2:======================================
1. # mkdir /dev/mapper
2. # virsh pool-start mpath
Pool mpath started

3. # service libvirtd restart
Redirecting to /bin/systemctl restart  libvirtd.service

4. # virsh pool-list --all
 Name                 State      Autostart
-------------------------------------------
...   
 mpath                active     no        <==== active as expected


Scenario 3:======================================
1. # rm -rf /dev/mapper/
2. # mkdir /dev/mpath
3. # virsh pool-start mpath
error: Failed to start pool mpath
error: Requested operation is not valid: storage pool 'mpath' is already active

4. # service libvirtd restart
Redirecting to /bin/systemctl restart  libvirtd.service
[root@localhost ~]# virsh pool-list --all
 Name                 State      Autostart
-------------------------------------------
...
 mpath                active     no        <====== active as expected
...

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2015-2202.html