core: organize AddVmCommand#canDoAction

Refactoring AddVmCommand#canDoAction method, such that it won't use the
local variable returnValue to store the result - instead, we return
false as soon as we detect a required condition that is false. those
changes make the code more readable and compact.

Change-Id: Ie4eda8868b50fd4a99462c0a649bba58e52f9bc0
Signed-off-by: Arik Hadas <ahadas@redhat.com>