xsm: clean up unneeded current references

xsm: clean up unneeded current references

Some XSM hooks in dummy.h used current->domain when this was also passed
as a parameter; use the parameter in these cases. There are two hooks
where this does not apply and which are not immediately obvious:
xsm_set_target's parameters are the device model and HVM domains, and
xsm_mem_sharing_op's first parameter is the source of the shared page,
not the domain making the hypercall.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>