core: Fix NPE in LiveMigrateVmDisksCommand

Commit ed336c64b281bac151cb365f84668f401f272afa
removes 'setStoragePoolId(getVm().getStoragePoolId())'
from getPermissionCheckSubjects() since it's
not needed for perms check anymore.
Hence, added it back in canDoAction to prevent NPE
on canDo validations.

Change-Id: Ic64bddc52fa5c1745c6064c8243f8c4e7921ef22
Signed-off-by: Daniel Erez <derez@redhat.com>