meson: Introduce qemu_datadir option

meson: Introduce qemu_datadir option

There is no guarantee that QEMU and libvirt have been configured
with the same prefix.

In particular, Homebrew on macOS will pass a different, private
prefix for each package version and then use symlinks to make
the files for a specific version appear in the usual locations.

This works perfectly fine as long as one package doesn't try to
go poking around another package's data - which is exactly what
libvirt needs to do in order to read and parse the QEMU interop
data.

qemu_datadir can now be explicitly provided to make this and
other uncommon scenarios work. The common scenario, where QEMU
and libvirt both use the same prefix, is unaffected.

https://gitlab.com/libvirt/libvirt/-/issues/168

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>