x86/HVM: grow MMIO cache data size to 64 bytes

x86/HVM: grow MMIO cache data size to 64 bytes

This is needed before enabling any AVX512 insns in the emulator. Change
the way alignment is enforced at the same time.

Add a check that the buffer won't actually overflow, and while at it
also convert the check for accesses to not cross page boundaries.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>