KVM: x86/xen: Use kvm_read_guest_virt() instead of open-coding it badly

KVM: x86/xen: Use kvm_read_guest_virt() instead of open-coding it badly

In particular, we shouldn't assume that being contiguous in guest virtual
address space means being contiguous in guest *physical* address space.

In dropping the manual calls to kvm_mmu_gva_to_gpa_system(), also drop
the srcu_read_lock() that was around them. All call sites are reached
from kvm_xen_hypercall() which is called from the handle_exit function
with the read lock already held.

Fixes: 2fd6df2f2 ("KVM: x86/xen: intercept EVTCHNOP_send from guests")
       536395260 ("KVM: x86/xen: handle PV timers oneshot mode")
       1a65105a5 ("KVM: x86/xen: handle PV spinlocks slowpath")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>