core: Re-ordering validations at create snapshot command.

Some validations in the create snapshot command were only executed if
the VM had disks, that caused the command to skip validations for
diskless VM.
The fix was to move the relevant validations to be executed regardless
to the VM disks.
Also added a test for create snapshot from VM.

Bug-Url: https://bugzilla.redhat.com/903245
Change-Id: I58bfc73383ce5ca06bf670a8f63b5563da9caa02
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>