x86/hvm: add check when register io handler

x86/hvm: add check when register io handler

At the time of registering HVM I/O handler, the HVM domain might
not have been initialized, which means the hvm_domain.io_handler
would be NULL. In the hvm_next_io_handler(), this should be asserted.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>