KVM: Use PCI device flag helper functions

Use PCI device flag helper functions when assigning or releasing device.
No functional change.

Signed-off-by: Ethan Zhao <ethan.zhao@oracle.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>