kvm-unit-tests: Add memcpy to lib/string.c

Add memcpy(void *dest, const void *src, size_t n) to lib/string.c.
This function acts the same as memcpy in libc.

Signed-off-by: Arthur Chunqi Li <yzt356@gmail.com>
[Add missing const. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>