[PATCH v3 4/7] elf_info: kcore: check for invalid physical address

kcore passes correct phys_start for direct mapped region and an invalid
value (-1) for all other regions after the kernel commit
464920104bf7(/proc/kcore: update physical address for kcore ram and
text). arch specific function is_phys_addr() accepts only virt_start.
Therefore, check for valid phys_start in get_kcore_dump_loads().

Signed-off-by: Pratyush Anand <panand@redhat.com>