Meanwhile both pkg-config and the perl build system cannot find the package

I've no experience of CentOS, but pkg-config will be looking for the file libvirt.pc, which would generally be in ../pkgconfig (relative to libvirt.a).

Did installing the devel package install a libvirt.pc ? (If not that would be a problem for pkg-config.)
Does pkg-config know where to find the .pc file ? (If not, then you set the PKG_CONFIG_PATH environment variable to the directory that houses the pc file ... and it should then find it.)

Also, in my experience, builds involving pkg-config work flawlessly so long as pkg-config can find the relevant pc file.

Cheers,
Rob