board/pc: add documentation for testing with qemu

Add some documentation about running the pc defconfigs in qemu.
In particular, document the use of the -bios parameter to use the OVMF
firmware to test the UEFI image.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>