x86/hvm: introduce functions for HVMOP_get/set_param allowance checks

x86/hvm: introduce functions for HVMOP_get/set_param allowance checks

Some parameters can only (validly) be set once. Some should not be set
by a guest for its own domain, and others must not be set since they
require the domain to be paused. Consolidate these checks, along with
the XSM check, in a new hvm_allow_set_param() function for clarity.

Also, introduce hvm_allow_get_param() for similar reasons.

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