meson: convert po/

Meson warns if xgettext is not found.  In the future we may want to add
a required argument to i18n.gettext(); in the meanwhile, I am adding a
--enable-gettext/--disable-gettext option and feature detection in
configure.  This preserves QEMU's default behavior of detecting system
features, without any warning, if neither --enable-* nor --disable-*
is requested.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>