oeqa/utils/qemurunner.py: Handle QEMU machines with a single serial

oeqa/utils/qemurunner.py: Handle QEMU machines with a single serial

Not all QEMU machines are capable of having more than one serial port,
this is due to the machine emulating a physical device/board.

Rework QemuRunner to handle machines that only have 1 serial port, where
the serial port shares output of the kernel log buffer and a login
console. In this case the output is mixed but enables the machine to
boot and have QemuRunner detect the login prompt.

QemuTarget uses SERIAL_CONSOLES to determine the number of available
serial ports.

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>