x86/VPMU: Initialize VPMU's lvtpc vector

x86/VPMU: Initialize VPMU's lvtpc vector

If a guest sets up performance counters so that they can generate
a PMC interrupt but does not initilaize APIC LVTPC register the
resulting interrupt will cause an APIC error.

Note that a guest deciding to clear LVTPC in order to unduce the error
will not be successful in achieving its goal: emulation code only
looks at the mask bit and always sets the vector to PMU_APIC_VECTOR.
Only the initial value of LVTPC (which is zero) that gets loaded into
APIC as result of PMC initialization is the problem.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>