bios_emulator: Add vesa support and allow ROMs to be passed in as data

As well as locating the ROM on the PCI bus, allow the ROM to be supplied to
the emulator. Split the init up a little so that callers can supply their
own interrupt routines. Also allow a vesa mode to be provided, to be
selected once the BIOS run is complete.

Signed-off-by: Simon Glass <sjg@chromium.org>