hw/i386/mshv: drop initial msi vector 0

This has been a warning before that was always raised if the machine has
a hpet. hpet_reset() will eventually result in mshv_send_msi called w/
vector 0, which we can safely drop.

Signed-off-by: Magnus Kulke <magnuskulke@linux.microsoft.com>
Link: https://lore.kernel.org/r/20260710101534.664604-13-magnuskulke@linux.microsoft.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>