pci: provide pci_enable_defaults()

One helper function to set/clear specific bit in PCI_COMMAND register is
introduced. Then, provide a function to do most of the common PCI init
work.

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