meson: fix keymaps without qemu-keymap

In case the qemu-keymap tool generating them is neither installed on the
system nor built from sources (due to xkbcommon not being available)
qemu will not find the keymaps when started directly from the build
tree,

This happens because commit ddcf607f ("meson: drop keymaps symlink")
removed the symlink to the source tree, and the special handling for
install doesn't help in case we do not install qemu.

Lets fix that by simply copying over the file from the source tree as
fallback.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20200827102617.14448-1-kraxel@redhat.com>
[thuth: Rebased, changed "config_host['qemu_datadir']" to "qemu_datadir",
        added Gerd's UNLINK fix to configure script]
Signed-off-by: Thomas Huth <thuth@redhat.com>