schema: Update VmDefinition to match current behavior

The schema for VmDefinition was incorrect in that it had some fields marked as
mandatory even though they may be omitted.  Also, exitCode and exitMessage were
not included in the original definition.

Signed-off-by: Adam Litke <agl@us.ibm.com>
Change-Id: I82e3b4e2073245fa837eb0333dfe1c90c7e4cfe8
Reviewed-on: http://gerrit.ovirt.org/11712
Reviewed-by: Saggi Mizrahi <smizrahi@redhat.com>