core: Cleanup VmPoolCommandBase#isVmFree

Cleaned up VmPoolCommandBase's isVmFree method to make it more readable:
1. Adopted an early-return paradigm
2. Removed redundant null checks on messages. This method is only called
   from canAttachNonPrestartedVmToUser, which always passes a new
   ArrayList instance.

Change-Id: I2e59cd811d7c318674587df877d29a10d8ef220f
Signed-off-by: Allon Mureinik <amureini@redhat.com>