pci: Only unmap bus_master_enabled_region if was added previously

Normally pci_init_bus_master() would be called either via
bus->machine_done.notify or directly from do_pci_register_device().

However if a device's realize() failed, pci_init_bus_master() is not
called, and do_pci_unregister_device() fails on
memory_region_del_subregion() as it was not mapped.

This adds a check that subregion was mapped before unmapping it.

Fixes: c53598ed18e4 ("pci: Add missing drop of bus master AS reference")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: John Snow <jsnow@redhat.com>
