qemu: add usb-serial support

qemu: add usb-serial support

Add an optional 'type' attribute to <target> element of serial port
device. There are two choices for its value, 'isa-serial' and
'usb-serial'. For backward compatibility, when attribute 'type' is
missing the 'isa-serial' will be chosen as before.

Libvirt XML sample

    <serial type='pty'>
      <target type='usb-serial' port='0'/>
      <address type='usb' bus='0' port='1'/>
    </serial>

qemu commandline:

qemu ${other_vm_args}              \
    -chardev pty,id=charserial0    \
    -device usb-serial,chardev=charserial0,id=serial0,bus=usb.0,port=1