xen: mask x2APIC feature in PV

On x2apic enabled pvm, doing sysrq+l, got NULL pointer dereference as below.

    SysRq : Show backtrace of all active CPUs
    BUG: unable to handle kernel NULL pointer dereference at           (null)
    IP: [<ffffffff8125e3cb>] memcpy+0xb/0x120
    Call Trace:
     [<ffffffff81039633>] ? __x2apic_send_IPI_mask+0x73/0x160
     [<ffffffff8103973e>] x2apic_send_IPI_all+0x1e/0x20
     [<ffffffff8103498c>] arch_trigger_all_cpu_backtrace+0x6c/0xb0
     [<ffffffff81501be4>] ? _raw_spin_lock_irqsave+0x34/0x50
     [<ffffffff8131654e>] sysrq_handle_showallcpus+0xe/0x10
     [<ffffffff8131616d>] __handle_sysrq+0x7d/0x140
     [<ffffffff81316230>] ? __handle_sysrq+0x140/0x140
     [<ffffffff81316287>] write_sysrq_trigger+0x57/0x60
     [<ffffffff811ca996>] proc_reg_write+0x86/0xc0
     [<ffffffff8116dd8e>] vfs_write+0xce/0x190
     [<ffffffff8116e3e5>] sys_write+0x55/0x90
     [<ffffffff8150a242>] system_call_fastpath+0x16/0x1b

That's because apic points to apic_x2apic_cluster or apic_x2apic_phys
but the basic element like cpumask isn't initialized.

Mask x2APIC feature in pvm to avoid overwrite of apic pointer,
update commit message per Konrad's suggestion.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Tested-by: Tamon Shiose <tamon.shiose@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>