Instead of calling a global function on coalesced mmio changes, which
routes the call to kvm if enabled, add coalesced mmio hooks to
MemoryListener and make kvm use that instead.

The motivation is support for multiple address spaces (which means we
we need to filter the call on the right address space) but the result
is cleaner as well.

Signed-off-by: Avi Kivity <avi@redhat.com>