xenfb: remove xen_init_display "temporary" hack

Initialize xenfb properly, as all other backends, from its own
"initialise" function.

Remove the dependency of vkbd on vfb: use qemu_console_lookup_by_index
to find the principal console (to get the size of the screen) instead of
relying on a vfb backend to be available (which adds a dependency
between the two).

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>