Keep track of the cached value of 'pm_eptgen' for each host cpu.

This is necessary to make sure that we invalidate the nested page table
TLB entries correctly when a vcpu moves between host cpus.
Deal with VM exits during event injection properly by re-injecting the event
on the next VM entry.
Add a debugging aid to get the page table entries that map a guest physical
address to a host physical address.

The option "--get-gpa-pmap <gpa>" for /usr/sbin/bhyvectl can be used to
inspect the mapping from the command line.