x86/hvm: make sure stdvga cache cannot be re-enabled

x86/hvm: make sure stdvga cache cannot be re-enabled

As soon as the cache is disabled, it will become out-of-sync with the
VGA device model and since no mechanism exists to acquire current VRAM
state from the device model, re-enabling it leads to stale data
being seen by the guest.

The problem was introduced by commit 3bbaaec0 ("x86/hvm: unify stdvga
mmio intercept with standard mmio intercept") and can be seen by
deliberately crashing a Windows guest; the BSOD output is corrupted.

This patch changes the existing 'cache' boolean in hvm_hw_stdvga into a
tri-state enum and only allows the state to move from 'uninitialized' to
'enabled'. Once the cache state becomes 'disabled' it will remain so for
the lifetime of the VM.

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