Put dependency files in proper subdir

This seems to resolve subtle breakages of our build system:

Dependency files generated for targets like 'dir/foo.o' were saved as
'foo.d'. Now, if there was also a target 'foo.o', one of the dependency
file was overwritten. Concrete example: libhw*/macio.o vs.
libhw*/ide/macio.o. And this often left a segfaulting build result
behind when changing the "wrong" data structures".

Fix it by generating proper 'dir/foo.d'.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>