KVM: vmx, svm: clean up mass updates to regs_avail/regs_dirty bits

Document the meaning of the three combinations of regs_avail and
regs_dirty.  Update regs_dirty just after writeback instead of
doing it later after vmexit.  After vmexit, instead, we clear the
regs_avail bits corresponding to lazily-loaded registers.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>