pci,net,pc enhancements

This includes some fixes and enhancements that accumulated in my tree:
pci fixes by dkoch, virtio-net enhancements by akong and mst,
and a fix for xen pc by mst.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

# gpg: Signature made Wed 17 Jul 2013 04:44:45 AM CDT using RSA key ID D28D5469
# gpg: Can't check signature: public key not found

# By Don Koch (2) and others
# Via Michael S. Tsirkin
* mst/tags/for_anthony:
  pc: don't access fw cfg if NULL
  virtio-net: add feature bit for any header s/g
  net: add support of mac-programming over macvtap in QEMU side
  pci: fix BRDIGE typo
  pci-bridge: update mappings for migration/restore

Message-id: 1374054430-21966-1-git-send-email-mst@redhat.com
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>