x86/monitor: disallow setting mem_access_emulate_each_rep when vm_event is NULL

x86/monitor: disallow setting mem_access_emulate_each_rep when vm_event is NULL

It is meaningless (and potentially dangerous - see hvmemul_virtual_to_linear())
to set mem_access_emulate_each_rep before xc_monitor_enable() (which allocates
vcpu->arch.vm_event) has been called, so return an error from the
XEN_DOMCTL_MONITOR_OP_EMULATE_EACH_REP hypercall when that is the case.

Signed-off-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citirx.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>