Description of problem:
Now virt-manager GUI can only support creating internal snapshots for vm which disk format is qcow2, but libvirt has support creating external snapshots for vm which disk format is qcow2, raw and so on, we hope virt-manager should also suppport it.

Version-Release number of selected component (if applicable):
virt-manager-1.3.2-1.el7.noarch
libvirt-1.3.4-1.el7.x86_64
kernel-3.10.0-421.el7.x86_64
qemu-kvm-rhev-2.6.0-4.el7.x86_64


How reproducible:
100%

Steps to Reproduce:
1. Check snapshots type created by virt-manager.
# virsh snapshot-info  rhel7.2 snapshot2 
Name:           snapshot2
Domain:         rhel7.2
Current:        yes
State:          running
Location:       internal--------<<
Parent:         snapshot1
Children:       0
Descendants:    0
Metadata:       yes

Result: virt-manager can only support creating internal snapshot for vm which disk type is qcow2.

2. Check libvirt manual page know external snapshots has been support.

# virsh snapshot-create-as rhel7.2 s3 --memspec /tmp/s3
Domain snapshot s3 created

# virsh snapshot-info rhel7.2 s3
Name:           s3
Domain:         rhel7.2
Current:        yes
State:          running
Location:       external-------------<<
Parent:         -
Children:       0
Descendants:    0
Metadata:       yes



Actual results:
virt-manager can only create internal snapshot for vm which disk format is qcow2.

Expected results:
virt-manager should support creating external snapshots for vm like libvirt

Additional info: