x86/hvm: remove hvm_io_pending() check in hvmemul_do_io()

x86/hvm: remove hvm_io_pending() check in hvmemul_do_io()

The check is done at the wrong point (since it is irrelevant if the
I/O is to be handled by the hypervisor) and its functionality can be
covered by returning X86EMUL_UNHANDLEABLE from hvm_send_assist_req()
instead.

This patch also removes the domain_crash() call from
hvm_send_assist_req(). Returning X86EMUL_UNHANDLEABLE allows the
higher layers of emulation to decide what to do instead.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>