xen/arm: Support hypercall_create_continuation for multicall

Multicall for ARM has been supported since commit f0dbdc6 "xen: arm: fully
implement multicall interface.". Although, if an hypercall in multicall
requires preemption, it will crash the host:

(XEN) Xen BUG at domain.c:347
(XEN) ----[ Xen-4.7-unstable  arm64  debug=y  Tainted:    C ]----
[...]
(XEN) Xen call trace:
(XEN)    [<00000000002420cc>] hypercall_create_continuation+0x64/0x380 (PC)
(XEN)    [<0000000000217274>] do_memory_op+0x1b00/0x2334 (LR)
(XEN)    [<0000000000250d2c>] do_multicall_call+0x114/0x124
(XEN)    [<0000000000217ff0>] do_multicall+0x17c/0x23c
(XEN)    [<000000000024f97c>] do_trap_hypercall+0x90/0x12c
(XEN)    [<0000000000251ca8>] do_trap_hypervisor+0xd2c/0x1ba4
(XEN)    [<00000000002582cc>] guest_sync+0x88/0xb8
(XEN)
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 5:
(XEN) Xen BUG at domain.c:347
(XEN) ****************************************
(XEN)
(XEN) Manual reset required ('noreboot' specified)

Looking to the code, the support of multicall looks valid to me, as we only
need to fill call.args[...]. So drop the BUG();

This is XSA-145.

Acked-by: Ian Campbell <ian.campbell@citrix.com>

diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
index 575745c..5c5ac9e 100644
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -344,8 +344,6 @@ unsigned long hypercall_create_continuation(
 
     if ( test_bit(_MCSF_in_multicall, &mcs->flags) )
     {
-        BUG(); /* XXX multicalls not implemented yet. */
-
         __set_bit(_MCSF_call_preempted, &mcs->flags);
 
         for ( i = 0; *p != '\0'; i++ )