x86/hypercall: Make the HVM hcall_preempted boolean common

x86/hypercall: Make the HVM hcall_preempted boolean common

HVM guests currently make use of arch.hvm_vcpu.hcall_preempted to track
hypercall preemption in struct vcpu.  Move this boolean to being common at the
top level of struct vcpu, which will allow it to be reused elsewhere.

Alter the PV preemption logic to use this boolean.  This simplifies the code
by removing guest-type-specific knowledge, and removes the risk of accidently
skipping backwards or forwards multiple times and corrupting %rip.

In pv_hypercall() the old_rip bodge can be removed, and parameter clobbering
can happen based on a more obvious condition.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>