x86/shadow: refactor shadow_vram_{get,put}_l1e()

x86/shadow: refactor shadow_vram_{get,put}_l1e()

By passing the functions an MFN and flags, only a single instance of
each is needed; they were pretty large for being inline functions
anyway.

While moving the code, also adjust coding style and add const where
sensible / possible.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>