xen/granttable: Disable grant v2 for HVM domains.

As proper scaffolding for supporting error status is not yet
implemented.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000400
IP: [<ffffffff81375ae9>] gnttab_end_foreign_access_ref_v2+0x29/0x40
PGD 32aa3067 PUD 32a87067 PMD 0
Oops: 0000 [#1] PREEMPT SMP
CPU 0
Modules linked in: sg sr_mod cdrom ata_generic ata_piix libata scsi_mod xen_blkfront xen_netfront fb_sys_fops sysimgblt sysfillrect syscopyarea xen_kbdfront
cmd

Pid: 2307, comm: ip Not tainted 3.3.0-rc1 #1 Xen HVM domU
RIP: 0010:[<ffffffff81375ae9>]  [<ffffffff81375ae9>] gnttab_end_foreign_access_ref_v2+0x29/0x40
RSP: 0018:ffff88003be03d38  EFLAGS: 00010206
RAX: 0000000000000000 RBX: ffff880033210640 RCX: 0000000000000040
RDX: 0000000000002000 RSI: 0000000000000000 RDI: 0000000000000200
RBP: ffff88003be03d38 R08: 0000000000000101 R09: 0000000000000000
R10: dead000000100100 R11: 0000000000000000 R12: ffff88003be03e48
R13: 0000000000000001 R14: ffff880039461c00 R15: 0000000000000200
FS:  00007fb1f84ec700(0000) GS:ffff88003be00000(0000) knlGS:0000000000000000
...

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>