tools: Fix configure of upstream QEMU

tools: Fix configure of upstream QEMU

QEMU as recently switch its build system to use meson and the
./configure step with meson is more restrictive that the step used to
be, most installation path wants to be within prefix, otherwise we
have this error message:

    ERROR: The value of the 'datadir' option is '/usr/share/qemu-xen' which must be a subdir of the prefix '/usr/lib/xen'.

In order to workaround the limitation, we will set prefix to the same
one as for the rest of Xen installation, and set all the other paths.

For reference, a thread in qemu-devel:
    "configure with datadir outside of --prefix fails with meson"
    https://lore.kernel.org/qemu-devel/20200918133012.GH2024@perard.uk.xensource.com/t/

And an issue in meson:
    "artificial limitation of directories (forced to be in prefix)"
    https://github.com/mesonbuild/meson/issues/2561

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Tested-by: Paul Durrant <paul@xen.org>
Acked-by: Wei Liu <wl@xen.org>