vmm: Fix AP startup compatibility for old bhyve executables

vmm: Fix AP startup compatibility for old bhyve executables

These changes unbreak AP startup when using a 13.1-RELEASE bhyve
executable with a newer kernel:
- Correct the destination mask for the VM_EXITCODE_IPI message generated
  by an INIT or STARTUP IPI in vlapic_icrlo_write_handler().
- Only initialize vlapics on active vCPUs.  13.1-RELEASE bhyve activates
  AP vCPUs only after the BSP starts them with an IPI, and vmm now
  allocates vcpu structures lazily, so the STARTUP handling in
  vm_handle_ipi() could trigger a page fault.
- Fix an off-by-one setting the vcpuid in a VM_EXITCODE_SPINUP_AP
  message.

Fixes:	7c326ab5bb9a ("vmm: don't lock a mtx in the icr_low write handler")
Reviewed by:	jhb, corvink
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D38446