x86/hvm: give HVMOP_set_param and HVMOP_get_param their own functions

x86/hvm: give HVMOP_set_param and HVMOP_get_param their own functions

The level of switch nesting in those ops is getting unreadable. Giving
them their own functions does introduce some code duplication in the
the pre-op checks but the overall result is easier to follow.

This patch is code movement (including style fixes). There is no
functional change.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>