ioreq-server: handle the lack of a default emulator properly

ioreq-server: handle the lack of a default emulator properly

I started porting QEMU over to use the new ioreq server API and hit a
problem with PCI bus enumeration. Because, with my patches, QEMU only
registers to handle config space accesses for the PCI device it implements
all other attempts by the guest to access 0xcfc go nowhere and this was
causing the vcpu to wedge up because nothing was completing the I/O.

This patch introduces an I/O completion handler into the hypervisor for the
case where no ioreq server matches a particular request. Read requests are
completed with 0xf's in the data buffer, writes and all other I/O req types
are ignored.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>