pci: Rework pci_bar_is_valid()

Cc: Thomas Huth <thuth@redhat.com>
Cc: Andrew Jones <drjones@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>