At least for x86 devices it is not necessary to provide these arguments.
Make them optional.

Add an x86 example to the documentation too.

Signed-off-by: Simon Glass <[email protected]>

These arguments are currently set on activation. Where the driver is
being used by a strategy, it may need to set the arguments after that.
There is no need to have everything set in stone until the on() method
is called, so move it there.

For existing users, no functional change is intended.

Signed-off-by: Simon Glass <[email protected]>

The U-Boot strategy will need to provide the filename of the BIOS
directly when booting U-Boot on QEMU. Add a method to support this.

Signed-off-by: Simon Glass <[email protected]>

Rather than provide a strange exception, complain when a strategy does
not use the QEMU driver properly, perhaps due to user settings.

Series-changes: 6
- Add new patch to report an error if QEMU is not turned on

Signed-off-by: Simon Glass <[email protected]>