KVM: VMX: cleanup vmx_set_cr0().

When calculating hw_cr0 teh current code masks bits that should be always
on and re-adds them back immediately after. Cleanup the code by masking
only those bits that should be dropped from hw_cr0. This allow us to
get rid of some defines.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>