memslot: Add support for READONLY mappings

A KVM memslot has a flags field, which allows to mark a region as
read-only.
Add another memory type bit to allow kvmtool-internal users to map a
write-protected region. Write access would trap and can be handled by
the MMIO emulation, which should register on the same guest address
region.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>