x86/HVM: improve MTRR load checks

x86/HVM: improve MTRR load checks

We should not assume that the incoming set of values contains exactly
MTRR_VCNT variable range MSRs. Permit a smaller amount and reject a
bigger one. As a result the save path then also needs to no longer use
a fixed upper bound, in turn requiring unused space in the save record
to be zeroed up front.

Also slightly refine types where appropriate.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
[switch to use MASK_EXTR to get VCNT]
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>