x86: streamline hypercall_create_continuation()

x86: streamline hypercall_create_continuation()

- drop clearing of excessive multicall arguments in compat case (no
  longer needed now that hypercall_xlat_continuation() only checks the
  actual arguments)
- latch current into a local variable
- use the cached value of hvm_guest_x86_mode() instead of re-executing
  it
- scope restrict "regs"
- while at it, convert the remaining two argument checking BUG_ON()s in
  hypercall_xlat_continuation() to ASSERT()s

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