xl: suppress suspend/resume functions on platforms which do not support it.

xl: suppress suspend/resume functions on platforms which do not support it.

ARM does not (currently) support migration, so stop offering tasty looking
treats like "xl migrate".

Apart from the UI improvement my intention is to use this in osstest to detect
whether to attempt the save/restore/migrate tests.

Other than the additions of the #define/#ifdef there is a tiny bit of code
motion ("dump-core" in the command list and core_dump_domain in the
implementations) which serves to put ifdeffable bits next to each other.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>