pcie: remove mmconfig memory leak and wrap mmconfig update with transaction

This memory leak was introduced inadvertently by omitting object_unparent.
A better fix is to use the new memory_region_set_size instead of destroying
and recreating the MMIO region on the fly.

Also, ensure that unmapping and remapping the region is done atomically.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
