x86/HVM: don't leak PFEC_implict to guests

x86/HVM: don't leak PFEC_implict to guests

Doing so may not only confuse them, but will - on VMX - lead to
VMRESUME failures. Add respective ASSERT()s where the fields get set
to guard against future similar issues (or - in the restore case -
fail the operation). In that latter code at once convert the mis-used
gdprintk() to dprintk(), as the vCPU of interest is not "current".

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