xen/arm: support gzip compressed kernels

xen/arm: support gzip compressed kernels

Free the memory used for the compressed kernel and update the relative
mod->start and mod->size parameters with the uncompressed ones.

To decompress the kernel, allocate memory from dommheap, because freeing
the modules is done by calling init_heap_pages, which frees to domheap.
Map these pages using vmap, because they might not be in the linear 1:1
map.

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