core: ExportVMCommand storage allocation checks

This patch is a part of a series of patches, adding storage allocation
validations to the system when they're missing, and replacing old
verification usage with unified, new, correct and tested verification.
This patch did this for ExportVmCommand, using only existing
validations.

Change-Id: Ic65fdc7324babacda2d1d49dc7157f6799b922f2
Bug-Url: https://bugzilla.redhat.com/1053747
Signed-off-by: Vered Volansky <vvolansk@redhat.com>