core: remove MigrateVmCommand#canMigrateVm

All that MigrateVmCommand#canDoAction was doing was to delegate to
canMigrateVm method and canMigrateVm method was not called from anywhere
else. Thus, this patch removes the canMigrateVm method and put its body
in the canDoAction method instead.

InternalMigrateVmCommand is also changed accordingly.

Change-Id: If88012b8bdbc7d23ad9350acf207431727e1d51e
Signed-off-by: Arik Hadas <ahadas@redhat.com>