qemu: snapshot: Forbid internal snapshot with passthrough devices

qemu: snapshot: Forbid internal snapshot with passthrough devices

When attempting to create internal system checkpoint with a passthrough
device qemu will report the following error:

error: operation failed: Error -22 while writing VM

This patch calls the function to check if migration is possible with
given VM and thus improves the error to:

error: Requested operation is not valid: domain has assigned non-USB host devices

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=874418#c19
Signed-off-by: Peter Krempa <pkrempa@redhat.com>