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>

meson.build

@@ -1685,6 +1685,12 @@ if not get_option('driver_qemu').disabled()
     endif
     conf.set_quoted('QEMU_MODDIR', qemu_moddir)
 
+    qemu_datadir = get_option('qemu_datadir')
+    if qemu_datadir == ''
+      qemu_datadir = datadir / 'qemu'
+    endif
+    conf.set_quoted('QEMU_DATADIR', qemu_datadir)
+
     if host_machine.system() in [ 'freebsd', 'darwin' ]
       default_qemu_user = 'root'
       default_qemu_group = 'wheel'