This patch changes coalesced mmio to create one mmio device per
zone instead of handling all zones in one device.

Doing so enables us to take advantage of existing locking and prevents
a race condition between coalesced mmio registration/unregistration
and lookups.
Suggested-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NSasha Levin <levinsasha928@gmail.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>