x86/HVM: clean up HVMOP_set_mem_type processing

x86/HVM: clean up HVMOP_set_mem_type processing

- drop unused variable "mfn"
- consistently do not use "else" when the prior "if" ends in "goto"
- use printk() referencing the target domain instead of gdprintk()
  (which references the current domain) and slightly shorten message
- annotate -EINVAL results in paging/shared paths to actually need
  switching to -EAGAIN (possible only when preemption logic got fixed
  to use -ERESTART)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>