xen/page_alloc: Remove dead code in alloc_domheap_pages()

xen/page_alloc: Remove dead code in alloc_domheap_pages()

Since commit 1aac966e24e9 "xen: support RAM at addresses 0 and 4096",
bits_to_zone() will never return 0 and it is expected that we have
minimum 2 zones.

Therefore the check in alloc_domheap_pages() is unnecessary and can
be removed. However, for sanity, it is replaced with an ASSERT().

Also take the opportunity to switch from min_t() to min() as
bits_to_zone() cannot return a negative value. The macro is tweaked
to make it clearer.

This bug was discovered and resolved using Coverity Static Analysis
Security Testing (SAST) by Synopsys, Inc.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>