Currently we format the serial, geometry and error policy on the -drive
backend argument.

QEMU added the ability to set serial and geometry on the frontend in
the 1.2 release deprecating use of -drive, with support being deleted
from -drive in 3.0.

We keep formatting error policy on -drive for now, because we don't
ahve support for that with -device for usb-storage just yet.

Note that some disk buses (sd) still don't support -device. Although
QEMU allowed these properties to be set on -drive for if=sd, they
have been ignored so we now report an error in this case.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>