x86/hvm: Improve grant_table_op hypercall dispatching

x86/hvm: Improve grant_table_op hypercall dispatching

hvm_grant_table_op() and hvm_grant_table_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_grant_table_op() (folding
grant_table_op_is_allowed() into is now-single caller) and dispatch to
{do,compat}_grant_table_op() based on the hcall_64bit setting.

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