dump: fill in the flat header signature more pleasingly to the eye

dump: fill in the flat header signature more pleasingly to the eye

The "mh.signature" array field has size 16, and is zeroed by the preceding
memset(). MAKEDUMPFILE_SIGNATURE expands to a string literal with string
length 12 (size 13). There's no need to measure the length of
MAKEDUMPFILE_SIGNATURE at runtime, nor for the extra zero-filling of
"mh.signature" with strncpy().

Use memcpy() with MIN(sizeof, sizeof) for robustness (which is an integer
constant expression, evaluable at compile time.)

Approximately-suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>