pci: Add pci_dev_exists()

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>
Message-Id: <bbe2afcf1701fa8905d910e141c922e2b058073f.1478512824.git.agordeev@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>