tests: add a a simple libvirt-is-version test tool

libvirt-is-version returns successfully in case the available version of
libvirt is greater or equal than the specified major/minor/release
values.
tests/disks: skip test-qemu-drive-libvirt.sh if libvirt is < 1.1.3

This test relies on the <test:runstate> element in the domain XML, which
has been introduced in libvirt 1.1.3.