x86/domctl: two functional fixes to XEN_DOMCTL_[gs]etvcpuextstate

x86/domctl: two functional fixes to XEN_DOMCTL_[gs]etvcpuextstate

Interacting with the vcpu itself should be protected by vcpu_pause().
Buggy/naive toolstacks might encounter adverse interaction with a vcpu context
switch, or increase of xcr0_accum.  There are no much problems with current
in-tree code.

Explicitly permit a NULL guest handle as being a request for size.  It is the
prevailing Xen style, and without it, valgrind's ioctl handler is unable to
determine whether evc->buffer actually got written to.

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