x86/hvm: Improve physdev_op hypercall dispatching

x86/hvm: Improve physdev_op hypercall dispatching

hvm_physdev_op() and hvm_physdev_op_compat32() are almost identical, but there
is no need to have two functions instantiated at the end of different function
pointers.

Combine the two into a single hvm_physdev_op() and dispatch to
{do,compat}_physdev_op() based on the hcall_64bit setting.

This also fixes an inconsistency where 64bit PVH hardware domains were
permitted access to extra physdev ops, but 32bit domains weren't.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>