frontend: disable memory field for running vm

For a VM which is not down, the memory size field will now be disabled
from the edit VM dialog.

Change-Id: Ic8902173adb1c20bd2cad11788a158ccbd5baf64
Bug-Url: https://bugzilla.redhat.com/1073479
Signed-off-by: Arik Hadas <ahadas@redhat.com>