hvm/vidirian: Avoid printing page_to_mfn(NULL) on error paths

hvm/vidirian: Avoid printing page_to_mfn(NULL) on error paths

While working in the viridian code, I noticed that 4cb6c4f4941

"x86/hvm: Use get_page_from_gfn() instead of get_gfn()/put_gfn."

introduced two error paths where page_to_mfn(NULL) would be formatted and
presented as a bad MFN.  This provides junk in the warning rather than
something useful.

These two codepaths are fixed up to match their counterpart in
wrmsr_hypervisor_regs()

While auditing the other changes from 4cb6c4f4941, I noticed a small
optimisation which could be made by changing the order of the validity checks
to remove 6 NULL pointer checks.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>