[Libvir] Please check my autoconf libvirt.m4

[Mark McLoughlin]

Hi Rich,
	I'd stick with just using pkg-config to check for the library itself
and using a runtime check for supported drivers.

	As for checking for the python module, I don't really think
virt-manager wants to do this at build time since it's not actually a
build dep. Personally I think I'd prefer an install time check for
python modules rather than runtime checks, but it seems that runtime
checking is the usual python way.

Cheers,
Mark.