Add host_memory_backend_pagesize() helper

Add host_memory_backend_pagesize() helper

There are a couple places (one generic, one target specific) where we need
to get the host page size associated with a particular memory backend.  I
have some upcoming code which will add another place which wants this.  So,
for convenience, add a helper function to calculate this.

host_memory_backend_pagesize() returns the host pagesize for a given
HostMemoryBackend object.

Signed-off-by: David Gibson <[email protected]>
Reviewed-by: Greg Kurz <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>