Posted a patch upstream to resolve this:

http://www.redhat.com/archives/libvir-list/2015-May/msg00865.html

Patch was pushed upstream:

commit 6839b08ba1e5e6dfe58d43a933d740198518e49d
Author: John Ferlan <jferlan>
Date:   Thu May 21 21:10:56 2015 -0400

    storage: Fix problem with disk backend pool allocation calculation

    The disk pool recalculates the pool allocation, capacity, and available
    values each time through processing a newly created disk partition. This
    created an issue with the allocation setting since the code used is shared
    with the refresh path. Each path calls virStorageBackendDiskReadPartitions
    which initializes the pool values and then processes the partition table
    from the 'libvirt_parthelper' utility output with the only difference being
    create passes a specific volume to be processed while refresh pass a NULL
    indicating to process all volumes. That passed volume is check during the
    virStorageBackendDiskMakeVol call to see if the current partition described
    by the volume key already exists. If it exists, then no adjustments are
    made to the allocation and the next entry in the output is checked.
    
    For the create path this resulted in only the most recently created
    partition size would be accounted for in the 'allocation' setting. This
    patch thus checks whether the incoming volume is NULL before clearing
    the pool allocation value.


git describe 6839b08ba1e5e6dfe58d43a933d740198518e49d
v1.2.16-rc1-16-g6839b08

PASS
Verified on libvirt-1.2.17-2.el7.x86_64

Please note, per bz 1206521 comment 8, extended disk won't take up pool's allocated space. 

Steps:
1. # cat disk.pool 
<pool type='disk'>
<name>sdb</name>
<source>
<device path='/dev/sdb'>
</device>
<format type='dos'/>
</source>
<target>
<path>/dev</path>
<permissions>
<mode>0700</mode>
<owner>0</owner>
<group>0</group>
</permissions>
</target>
</pool>

2. #virsh pool-define disk.pool; virsh pool-build sdb; virsh pool-start sdb
3. # virsh pool-info sdb
Name:           sdb
UUID:           0d5e0da6-db87-4a93-975e-4d3467a64d4a
State:          running
Persistent:     yes
Autostart:      no
Capacity:       7.45 GiB
Allocation:     0.00 B
Available:      7.45 GiB

4. # virsh vol-create-as sdb sdb1 2G  <=== create a primary partition
Vol sdb1 created

5. # virsh pool-info sdb
Name:           sdb
UUID:           0d5e0da6-db87-4a93-975e-4d3467a64d4a
State:          running
Persistent:     yes
Autostart:      no
Capacity:       7.45 GiB
Allocation:     2.00 GiB  <=== expected
Available:      5.45 GiB

6. # virsh vol-create-as sdb sdb2 2G  <=== create another partition
Vol sdb2 created

7. # virsh pool-info sdb
Name:           sdb
UUID:           0d5e0da6-db87-4a93-975e-4d3467a64d4a
State:          running
Persistent:     yes
Autostart:      no
Capacity:       7.45 GiB
Allocation:     4.00 GiB    <==== expected
Available:      3.45 GiB

8. #  virsh vol-create-as sdb sdb3 1G --format extended  <=== create a extended partition
Vol sdb3 created

9. # virsh pool-info sdb
Name:           sdb
UUID:           0d5e0da6-db87-4a93-975e-4d3467a64d4a
State:          running
Persistent:     yes
Autostart:      no
Capacity:       7.45 GiB
Allocation:     4.00 GiB  <==== extended partition won't take up allocation as expected
Available:      3.45 GiB

10. # virsh vol-list --details sdb
 Name  Path       Type   Capacity  Allocation
----------------------------------------------
 sdb1  /dev/sdb1  block  2.00 GiB    2.00 GiB
 sdb2  /dev/sdb2  block  2.00 GiB    2.00 GiB
 sdb3  /dev/sdb3  block  1.00 GiB    1.00 GiB  <===== extended partition can be viewed here as expected. 


NB. there is a bug for prated bz 1245144 which may cause some error message during vol-create-as, but partitions could be successfully created.

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