qemu: split out qemu-guest-agent, add startup scripts

qemu: split out qemu-guest-agent, add startup scripts

Split out the QEMU guest agent into a separate package. The agent is
intended to be installed within a QEMU VM guest where a user is likely
to not want to have the rest of the QEMU installation within it.

Additionally, an initscript, udev rules file, and systemd unit file are
added to the package so that the guest agent can start automatically;
the former two come from Debian's packaging for qemu-guest-agent.

Signed-off-by: Brenda Streiff <brenda.streiff@ni.com>
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>