Add the port allocation logic for isa-serial devices.

Add the port allocation logic for isa-serial devices.

This commit takes care of following cases:
-> Check availability of requested ports.
  ->The total number of requested ports should not be more than
    VIR_MAX_ISA_SERIAL_PORTS.
  ->The ports requested should be less than VIR_MAX_ISA_SERIAL_PORTS.
  ->VIR_MAX_ISA_SERIAL_PORTS should correspond to MAX_ISA_SERIAL_PORTS
    specified in QEMU code commit def337ffda34d331404bd7f1a42726b71500df22.
-> Prevent duplicate device assignments to the same port.
-> In case no ports are provided in the XML, this patch scans the list of unused
   isa-serial indices to automatically assign available ports for this VM.

Signed-off-by: Divya Garg <divya.garg@nutanix.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>