Simplify APIC mode switching from MMIO to x2APIC. In part this is done to
simplify the implementation of the x2APIC virtualization assist in VT-x.

Prior to this change the vlapic allowed the guest to change its mode from
xAPIC to x2APIC. We don't allow that any more and the vlapic mode is locked
when the virtual machine is created. This is not very constraining because
operating systems already have to deal with BIOS setting up the APIC in
x2APIC mode at boot.

Fix a bug in the CPUID emulation where the x2APIC capability was leaking
from the host to the guest.

Ignore MMIO reads and writes to the vlapic in x2APIC mode. Similarly, ignore
MSR accesses to the vlapic when it is in xAPIC mode.

The default configuration of the vlapic is xAPIC. The "-x" option to bhyve(8)
can be used to change the mode to x2APIC instead.

Discussed with: grehan@