x86/hvm: fix BUFIOREQ evtchn init for a stubdom

x86/hvm: fix BUFIOREQ evtchn init for a stubdom

This is a missing part from the previous patch that add the BUFIOREQ_EVTCHN
parameter. This patch changes the ownership of the buifioreq event channel to
the stubdom (when HVM_PARAM_DM_DOMAIN is set within the stubdom).

This patch introduces an helper to replace a xen port.

This fix the initialization of QEMU inside the stubdomain.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Add error handling to first invocation of hvm_replace_event_channel().
Adjust type of hvm_replace_event_channel()'s second parameter.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Committed-by: Jan Beulich <jbeulich@suse.com>