core: extract constant from HibernateVmCommand

META_DATA_SIZE_IN_BYTES was extracted to MemoryUtils due to use in
several vm related commands, as well as need for it in another command
in bz1119022 fix.

Change-Id: I1604cf4a82e3e202093acdab97939427ab335169
Bug-Url: https://bugzilla.redhat.com/1119022
Signed-off-by: Vered Volansky <vvolansk@redhat.com>