Fix race in finding available vnc port

Fix race in finding available vnc port

The qemu driver contains a subtle race in the logic to find next
available vnc port.  Currently it iterates through all available ports
and returns the first for which bind(2) succeeds.  However it is possible
that a previously issued port has not yet been bound by qemu, resulting
in the same port used for a subsequent domain.

This patch addresses the race by using a simple bitmap to "reserve" the
ports allocated by libvirt.

V2:
  - Put port bitmap in struct qemud_driver
  - Initialize bitmap in qemudStartup

V3:
  - Check for failure of virBitmapGetBit
  - Additional check for port != -1 before calling virbitmapClearBit

V4:
  - Check for failure of virBitmap{Set,Clear}Bit