x86 pv: Crash when handling guest access to MSR_MISC_ENABLE

ISSUE DESCRIPTION

When a guest accesses certain Model Specific Registers, Xen first reads the value from hardware to use as the basis for auditing the guest access.
For the MISC_ENABLE MSR, which is an Intel specific MSR, this MSR read is performed without error handling for a #GP fault, which is the consequence of trying to read this MSR on non-Intel hardware.

IMPACT

A buggy or malicious PV guest administrator can crash Xen, resulting in a host Denial of Service.

VULNERABLE SYSTEMS

Only x86 systems are vulnerable. ARM systems are not vulnerable.
Only Xen versions 4.11 and onwards are vulnerable. 4.10 and earlier are not vulnerable.
Only x86 systems which do not implement the MISC_ENABLE MSR (0x1a0) are vulnerable. AMD and Hygon systems do not implement this MSR and are vulnerable. Intel systems do implement this MSR and are not vulnerable. Other manufacturers have not been checked.
Only x86 PV guests can exploit the vulnerability. x86 HVM/PVH guests cannot exploit the vulnerability.