VMX: use xAPIC mode on all processors

Results are undefined if xAPIC/x2APIC mode is not homogeneous on all processors.
So far things seemed to have mostly worked, but if you end up calling xapic_icr_write
from an x2APIC-mode processor the write is eaten and the IPI is not delivered.

Reported-by: Cathy Avery <cavery@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>