Documentation/hw-vuln: Add VMSCAPE documentation

Documentation/hw-vuln: Add VMSCAPE documentation

VMSCAPE is a vulnerability that may allow a guest to influence the branch
prediction in host userspace, particularly affecting hypervisors like QEMU.

Add the documentation.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>