[libvirt] handle different version of QEMU

[Matthias Gatto]

Hello,

I'm implementing the "new" QEMU option of
block_set_io_throttle(@bps_max, @bps_rd_max...), but i have a problem:
The old version of QEMU(before 1.7) does not handle it, so I'd like to
know what function should i use to know the version of QEMU, and
what's the proper way to handle this problem ?