vmx: Don't enable irq for machine check vmexit handling

vmx: Don't enable irq for machine check vmexit handling

We should not enable irq for machine check VMExit

In changeset 18658:824892134573, IRQ is enabled during VMExit except
external interrupt. The exception should apply for machine check also,
because :
a) The mce_logout_lock should be held in irq_disabled context.
b) The machine check event should be handled as quickly as possible,
enable irq will increase the period greatly.

Signed-off-by: Jiang, Yunhong <yunhong.jiang@intel.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>