Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC 257422,257661,258075,258476,258494,258579,258609,258699:
Several enhancements to the I/O APIC support in bhyve including:
- Move the I/O APIC device model from userspace into vmm.ko and add
ioctls to assert and deassert I/O APIC pins.
- Add HPET device emulation including a single timer block with 8 timers.
- Remove the 'vdev' abstraction.

Approved by: neel

Add HPET device emulation to bhyve.

bhyve supports a single timer block with 8 timers. The timers are all 32-bit
and capable of being operated in periodic mode. All timers support interrupt
delivery using MSI. Timers 0 and 1 also support legacy interrupt routing.

At the moment the timers are not connected to any ioapic pins but that will
be addressed in a subsequent commit.

This change is based on a patch from Tycho Nightingale (tycho.nightingale@pluribusnetworks.com).