MSI-X works slightly different than INTx; the doorbell
registers are not necessarily used as MSI-X interrupts
are directed anyway. So the head pointer on the
reply queue needs to be updated as soon as a frame
is completed, and we can set the doorbell only
when in INTx mode.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>