meson-buildoptions: add support for string options

meson-buildoptions: add support for string options

Allow using the buildoptions.json file for more options, namely anything
that is not a boolean or multiple-choice.

The mapping between configure and meson is messy for string options,
so allow configure to use to something other than the name in
meson_options.txt.  This will come in handy anyway for builtin
Meson options such as b_lto or b_coverage.

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