disk: serial no must now be passed to device not drive

disk: serial no must now be passed to device not drive

With QEMU 2.10 the serial parameter of the -drive command line option
was deprecated [1], so move the logic which adds this parameter now
to the -drive analogue -device CLI option.

Features marked deprecated will continue to work for two releases[2],
so we need to switch over before 2.12, AFAICT.

[1]: https://wiki.qemu.org/ChangeLog/2.10#Deprecated_options
[2]: https://qemu.weilnetz.de/doc/qemu-doc.html#Deprecated-features

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>