qemu: Reject attempts to create snapshots with names containig '/'
qemu: Reject attempts to create snapshots with names containig '/'

https://bugzilla.redhat.com/show_bug.cgi?id=896403

The snapshot name is used to create path to the definition save file.
When the name contains slashes the creation of the file fails. Reject
such names.
(cherry picked from commit 790f912b468657859466215312dbfb0b1a9a1f69)

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