lib/classes/conf: refactor qemu.bbclass functions into library functions

lib/classes/conf: refactor qemu.bbclass functions into library functions

Move the functions in qemu.bbclass to meta/lib/oe/qemu.py as they are
generally useful.

The qemu.bbclass is still kept, and recipes can continue to use functions
from it, though they have become wrapper functions on qemu.py functions.

Note that the QEMU_OPTIONS settings are still kept in qemu.bbclass.
This sets a clear barrier for people to use qemu user mode.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>