keyhandler: rework keyhandler infrastructure

keyhandler: rework keyhandler infrastructure

struct keyhandler does not contain much information, and requires a lot
of boilerplate to use.  It is far more convenient to have
register_keyhandler() take each piece of information a parameter,
especially when introducing temporary debugging keyhandlers.

This in turn allows struct keyhandler itself to become private to
keyhandler.c and for the key_table to become more efficient.

key_table doesn't need to contain 256 entries; all keys are ASCII which
limits them to 7 bits of index, rather than 8.  It can also become a
straight array, rather than an array of pointers.  The overall effect of
this is the key_table grows in size by 50%, but there are no longer
24-byte keyhandler structures all over the data section.

All of the key_table entries in keyhandler.c can be initialised at
compile time rather than runtime.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>