libxl: Implement domain metadata getter/setter
libxl: Implement domain metadata getter/setter

Fortunately, the code that handles metadata getting or setting is
driver agnostic, so all that is needed from individual hypervisor
drivers is to call the right functions.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1732306

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Erik Skultety <2c14d38fa47c8799f1b9c16280abe27f8edfec6e@redhat.com>

libvirt/libvirt,zippy2/libvirt,crobinso/libvirt,fabianfreyer/libvirt,crobinso/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,olafhering/libvirt,nertpinx/libvirt,jardasgit/libvirt,olafhering/libvirt,andreabolognani/libvirt,jardasgit/libvirt,libvirt/libvirt,jfehlig/libvirt,libvirt/libvirt,fabianfreyer/libvirt,olafhering/libvirt,fabianfreyer/libvirt,zippy2/libvirt,andreabolognani/libvirt,zippy2/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,jardasgit/libvirt,nertpinx/libvirt,jardasgit/libvirt,nertpinx/libvirt,zippy2/libvirt,olafhering/libvirt,libvirt/libvirt,crobinso/libvirt,jfehlig/libvirt,jardasgit/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,nertpinx/libvirt,jfehlig/libvirt,crobinso/libvirt