engine: Optimyzing RemoveVds

The following patch will improve canRemoveVds method of RemoveVdsCommand.
The change will reduce a number of queries and will make code more clear.
Also removed duplicate check inside execute() - it will not prevent a race,
but it always reduce performance

Change-Id: Id2a11727041955aeda1940fca1f27c616f46135f
Signed-off-by: Michael Kublin <mkublin@redhat.com>
engine: Fixing small bug in canDoaction() of RemoveVdsCommand

Missing condition in if, change is obvious

Change-Id: I7f342d15a7c9f2d6e6ed8de05ab87b839600c56b
Signed-off-by: Michael Kublin <mkublin@redhat.com>