x86/vmx: Don't self-recurse in vmx_update_guest_cr()

x86/vmx: Don't self-recurse in vmx_update_guest_cr()

An update to CR4 following a CR0 update can be done easily by falling
through into the CR4 case.  This avoids unnecessary passes through
vmx_vmcs_{enter,exit}() and unnecessary stack usage (as the compiler
cannot optimise this use to a tailcall).

No behavioural change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>