KVM: selftests: Add back the missing check of MONITOR/MWAIT availability

KVM: selftests: Add back the missing check of MONITOR/MWAIT availability

The revamp of monitor/mwait test missed the original check of feature
availability [*]. If MONITOR/MWAIT is not supported or is disabled by
IA32_MISC_ENABLE on the host, executing MONITOR or MWAIT instruction
from guest doesn't cause monitor/mwait VM exits, but a #UD.

[*] https://lore.kernel.org/all/20240411210237.34646-1-zide.chen@intel.com/

Reported-by: Xuelian Guo <xuelian.guo@intel.com>
Fixes: 80fd663590cf ("selftests: kvm: revamp MONITOR/MWAIT tests")
Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Link: https://lore.kernel.org/r/20250620062219.342930-1-chenyi.qiang@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>