Description of problem:
auto_dump_path setting in the qemu.conf does not work, the auto-dump file will be created in the '/' directory.

Version-Release number of selected component (if applicable):
libvirt-2.0.0-2.el7.x86_64
qemu-kvm-rhev-2.6.0-12.el7.x86_64


How reproducible:
100%

Steps to Reproduce:
1.Check the default setting of the auto_dump_path:
  #cat /etc/libvirt/qemu.conf
    ...
    # When a domain is configured to be auto-dumped when libvirtd receives a
    # watchdog event from qemu guest, libvirtd will save dump files in directory
    # specified by auto_dump_path. Default value is /var/lib/libvirt/qemu/dump
    #
    #auto_dump_path = "/var/lib/libvirt/qemu/dump"
     ...

2.Start a guest with watchdog device:
   #virsh dumpxml rhel7.3
    ...
    <watchdog model='i6300esb' action='dump'>
      <alias name='watchdog0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0c' function='0x0'/>
    </watchdog>
   ...

3.In guest:
  echo 1 > /dev/watchdog

Actual results:
The auto dump file created in the '/' directory:

Expected results:
The auto dump file created in the default auto_dump_path: /var/lib/libvirt/qemu/dump

Additional info: 
The dump filename is only consists of 'domainid-domainname', so the dump filename is the same for the same guest, it means the dumpfile will be overlapped by the next dump.