vmw_pvscsi: check message ring page count at initialisation

A guest could set the message ring page count to zero, resulting in
infinite loop. Add check to avoid it.

Reported-by: YY Z <bigbird475958471@gmail.com>
Signed-off-by: P J P <ppandit@redhat.com>
Message-Id: <20170425130623.3649-1-ppandit@redhat.com>
Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
