QEMU balloon script abstraction improvement

[slakkala]

Make the QEMU balloon script abstract away the difference between hypervisor balloon commands. The QMP balloon command expects the wanted memory size of the guest, so calculate the correct value from the balloon size given as argument.

Also some changes to reported balloon size; the changed signal is not emitted, if balloon size is not changed; and may report an old value.

[slakkala]

Added a description of SIZE. Also added balloon size query to cloud-hv's balloon script.

[mikatammi]

[astro]

Deprecated by #343