arm: compile libxenguest

arm: compile libxenguest

Introduce an empty implementation of the arch specific ARM functions in
xc_dom_arm.c.
Provide empty implementations of xc_domain_save and xc_domain_restore
when CONFIG_MIGRATE is not set.
Move xc_hvm_build.c to xc_hvm_build_x86.c because the implementation is
x86 specific, introduce xc_hvm_build_arm.c with empty stubs.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>