libvmmapi: Make vm_raise_msi() a common function

Currently, bhyve PCI emulation uses vm_lapic_msi() to raise an MSI in
the guest. The arm64 port has a similar function, vm_raise_msi().
Add vm_raise_msi() on amd64 as well and have it simply call
vm_lapic_msi() so that bhyve can use a common, generically named
function.

Reviewed by: corvink, andrew, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D41752