kvm: vmx: rename vmx_pre/post_block to pi_pre/post_block

Prepare to switch from preemption timer to hrtimer in the
vmx_pre/post_block. Current functions are only for posted interrupt,
rename them accordingly.

Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>