snapshot: qemu: Remove restrictions preventing external checkpoints
snapshot: qemu: Remove restrictions preventing external checkpoints

https://bugzilla.redhat.com/show_bug.cgi?id=638512

Some of the pre-snapshot check have restrictions wired in regarding
configuration options that influence taking of external checkpoints.

This patch removes restrictions that would inhibit taking of such a
snapshot.
(cherry picked from commit 0dac29d89f9541d8054e28fddfa7033526e297ba)

mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2