core: fetch disks internally in RunVmValidator

RunVmValidator was used to get the disks of the VM which is going to be
run from outside, as a parameter of the canRunVm method. This patch
changes this, such that the disks are no longer passed from outside,
they are now fetched internally.

Change-Id: I0c06577a1fab8c3c912f91a78a4303f83c8edcc0
Signed-off-by: Arik Hadas <ahadas@redhat.com>