tests/boot-serial-test: Add code to allow to specify our own kernel or bios

QEMU only ships with some few firmware images, i.e. we can currently run
the boot-serial test only on a very limited set of machines. But writing
some characters to the default UART of a machine can often be done with
some few lines of assembly, so we add the possibility to the boot-serial
tester to use its own mini-kernels or mini-firmwares. We write such images
then into a file that we can load with the "-kernel" or "-bios" parameter
when we launch QEMU.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1512031988-32490-3-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
