serial-pci: remove memory regions from BAR before destroying them

Otherwise, hot-unplug of pci-serial-2x trips the assertion
in memory_region_destroy:

    (qemu) device_del gg
    (qemu) qemu-system-x86_64: /work/armbru/tmp/qemu/memory.c:1021: memory_region_destroy: Assertion `((&mr->subregions)->tqh_first == ((void *)0))' failed.
    Aborted (core dumped)

Reported-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>