exec.c: Fix subpage memory access to RAM MemoryRegion

Commit 95c318f5 (Fix segfault in mmio
subpage handling code.) prevented a segfault by making all subpage
registrations over an existing memory page perform an unassigned access.
Symptoms were writes not taking effect and reads returning zero.

Very small page sizes are not currently supported either,
so subpage memory areas cannot fully be avoided.

Therefore change the previous fix to use a new IO_MEM_SUBPAGE_RAM
instead of IO_MEM_UNASSIGNED. Suggested by Avi.

Reviewed-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Cc: Avi Kivity <avi@redhat.com>
Cc: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>