Fix segfault in mmio subpage handling code.

It is possible that subpage mmio is registered over existing memory
page. When this happens "memory" will have real memory address and not
index into io_mem array so next access to the page will generate
segfault. It is uncommon to have some part of a page to be accessed as
memory and some as mmio, but qemu shouldn't crash even when guest does
stupid things. So lets just pretend that the rest of the page is
unassigned if guest configure part of the memory page as mmio.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
