x86/mm: add pv prefix to {set,destroy}_gdt

x86/mm: add pv prefix to {set,destroy}_gdt

Unfortunately they can't stay local to PV code because domain.c still
needs them. Change their names and fix up call sites. The code will be
moved later together with other descriptor table manipulation code.

Also move the declarations to pv/mm.h and provide stubs.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>