VMX: reduce number of posted-interrupt hooks

VMX: reduce number of posted-interrupt hooks

Three of the four hooks are not exposed outside of vmx.c, and all of
them have only a single possible non-NULL value. So there's no reason to
use hooks here - a simple set of flag indicators is sufficient (and we
don't even need a flag for the VM entry one, as it's always
(de-)activated together the the vCPU blocking hook, which needs to
remain an actual function pointer). This is the more that with the
Spectre v2 workarounds indirect calls have become more expensive.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>