ioport: reserve the whole range of an I/O port in the AddressSpace

When an I/O port is more than 1 byte long, ioport.c is currently
creating "short" regions, for example 0x1ce-0x1ce for the 16-bit
Bochs index port.  When I/O ports are memory mapped, and thus
accessed via a subpage_ops memory region, subpage_accepts gets
confused because it finds a hole at 0x1cf and rejects the access.

In order to fix this, modify registration of the region to cover
the whole size of the I/O port.  Attempts to access an invalid
port will be blocked by find_portio returning NULL.

This only affects the VBE DISPI regions.  For all other cases,
the MemoryRegionPortio entries for 2- or 4-byte accesses overlap
an entry for 1-byte accesses, thus the size of the memory region
is not affected.

Reported-by: Zoltan Balaton <balaton@eik.bme.hu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
