snapshot: fix state after external snapshot of S3 domain

snapshot: fix state after external snapshot of S3 domain

https://bugzilla.redhat.com/show_bug.cgi?id=876829 complains that
if a guest is put into S3 state (such as via virsh dompmsuspend)
and then an external snapshot is taken, qemu forcefully transitions
the domain to paused, but libvirt doesn't reflect that change
internally.  Thus, a user has to use 'virsh suspend' to get libvirt
back in sync with qemu state, and if the user doesn't know this
trick, then the guest appears hung.

* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateActiveExternal):
Track fact that qemu wakes up a suspended domain on migration.