Use boot_el to detect hypervisor [patch in bugzilla]

I've sent a patch in
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=292082 to add a check in
machdep.c to use the boot_el = EL1 and presence of EL2 capabilities to
detect that we're running under (some) hypervisor, which is taking EL2 for
itself.

This feels a little bit cleaner than relying solely on smbios strings. It
doesn't tell us anything about the hypervisor, but we'll know more reliably
that there is one there.

I've tested this on c4a VMs on GCE where I'm using it for an "enable MSI if
we're a VM" PCI quirk that I'll submit separately. (Because nvme0 without
interrupts is having a real sad day.) I don't have any other arm64 hardware
available to me to test with.
-- 
wac