x86, vmi: Fix vmi_get_timer_vector() to use IRQ0_VECTOR

FIRST_DEVICE_VECTOR is going away and it looks like a bad hack to steal
FIRST_DEVICE_VECTOR / FIRST_EXTERNAL_VECTOR, when it looks like it needs
IRQ0_VECTOR.

Fix vmi_get_timer_vector() to use IRQ0_VECTOR.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <20100114002118.436172066@sbs-t61.sc.intel.com>
Cc: Alok N Kataria <akataria@vmware.com>
Cc: Zach Amsden <zach@vmware.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>