x86/HVM: add new functions to get/set memory types

x86/HVM: add new functions to get/set memory types

For clarity this patch breaks the code to set/get memory types out
of do_hvm_op() into dedicated functions: hvmop_set/get_mem_type().
Also, for clarity, checks for whether a memory type change is allowed
are broken out into a separate function called by hvmop_set_mem_type().

There is no intentional functional change in this patch.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>