memory: remove MemoryRegion::backend_registered

backend_registered was used to lazify the process of registering an
mmio region, since the it is different for the I/O address space and
the memory address space.  However, it also makes registration dependent
on the region being visible in the address space.  This is not the case
for "fake" regions, like watchpoints or IO_MEM_UNASSIGNED.

Remove backend_registered and always initialize the region.  If it turns
out to be part of the I/O address space, we've wasted an I/O slot, but
that's not too bad.  In any case this will be optimized later on.

Signed-off-by: Avi Kivity <avi@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
