pci: provide pci_scan_bars()

Let's provide a more general way to scan PCI bars, rather than read the
config registers every time.

Then let x86/vmexit.c leverage pci_scan_bars()

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>