remove redundant libvirt call

Checking UUIDString() adresses the bug #603494, which
is closed ages ago. While it is nice to support really
ancient libvirt versions, it costs us one extra
libvirt call. Given global libvirt driver lock, it
does matter in mass VM creation.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=603494
Signed-off-by: Peter V. Saveliev <peet@redhat.com>
Change-Id: I06a71c2e44332f50b3203a0a8f1bfa799914b684
Reviewed-on: http://gerrit.ovirt.org/11054
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Adam Litke <alitke@redhat.com>