Add ELF header checking to prevent processing input file which is not
QEMU x86_64 guest memory dump or even not ELF.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1013



Signed-off-by: Viktor Prutyanov <viktor.prutyanov@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220520084339.171684-1-viktor.prutyanov@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>