KVM: x86: Add a VALID_MASK for the MSR exit reason flags

Add the mask KVM_MSR_EXIT_REASON_VALID_MASK for the MSR exit reason
flags.  This simplifies checks that validate these flags, and makes it
easier to introduce new flags in the future.

No functional change intended.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Message-Id: <20220921151525.904162-3-aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>