KVM: fix per-instance stored UID value

When using the pool security model, _ExecuteKVMRuntime was storing the
instance's UID using str(uid), which would result in storing the
LockedUid.__repr__() result:

 $ cat /var/run/ganeti/kvm-hypervisor/uid/xxxxxxxxxxxxx
 <ganeti.uidpool.LockedUid object at 0x1f30610>

This patch restores the intended behaviour, by using LockedUid.AsStr().

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>