janitor: do not rely on indirect inclusions of or from qemu-char.h

Various header files rely on qemu-char.h including qemu-config.h or
main-loop.h, but they really do not need qemu-char.h at all (particularly
interesting is the case of the block layer!).  Clean this up, and also
add missing inclusions of qemu-char.h itself.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
