Input: xen-kbdfront - add module parameter for setting resolution

Add a parameter for setting the resolution of xen-kbdfront in order to
be able to cope with a (virtual) frame buffer of arbitrary resolution.

While at it remove the pointless second reading of parameters from
Xenstore in the device connection phase: all parameters are available
during device probing already and that is where they should be read.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>