core: organize can-do methods

AddVmCommand#canAddVm:
1. Return false with an error as soon as a problem is detected instead
of using the 'returnValue' field to store the result
2. Inline the 'exists' variable

AddVmFromScratchCommand#canDoAction:
1. Return false with an error as soon as a problem is detected instead
of using the 'result' field to store the result
2. Remove redundant addition of action messages
VdcBllMessages.VAR__ACTION__ADD and VdcBllMessages.VAR__TYPE__VM as they
are already added by AddVmCommand

Change-Id: I4589fa4681913e41b1be595131686932d92ca062
Signed-off-by: Arik Hadas <ahadas@redhat.com>