engine: Guaranteed memory not propagated to export storage

Min. allocated memory value for VM template not stored in export
storage domain fixed.

Storing of min. allocated memory value for VM template extracted
to common code base shared with VM export (XML transfer format).

Change-Id: Idc06026d1dc92236fe2a25c3bde25023ad4d5861
Signed-off-by: Libor Spevak <lspevak@redhat.com>