util: Add qemu_guest_getrandom and associated routines

util: Add qemu_guest_getrandom and associated routines

This routine is intended to produce high-quality random numbers to the
guest.  Normally, such numbers are crypto quality from the host, but a
command-line option can force the use of a fully deterministic sequence
for use while debugging.

Reviewed-by: Laurent Vivier <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>