core: Normalize default VM time zone representation

Added db migration script to update old VMs from engine versions which
used empty string to represent 'Use default TimeZone' to value of NULL
which is used currently.

Change-Id: Ia5ffcfef3b2f1930e65c2f66e26c265df31e0813
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=988259
Signed-off-by: Martin Betak <mbetak@redhat.com>