qdev: Split up header so it can be used in cpu.h

Header file dependency is a frickin' nightmare right now.  cpu.h tends
to get included in our 'include everything' header files but qdev also
needs to include those headers mainly for qdev-properties since it knows
about CharDriverState and friends.

We can solve this for now by splitting out qdev.h along the same lines
that we previously split the C file.  Then cpu.h just needs to include
qdev-core.h.

hw/qdev.h is split into following new headers:
    hw/qdev-core.h
    hw/qdev-properties.h
    hw/qdev-monitor.h

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

[ehabkost: re-add DEFINE_PROP_PCI_HOST_DEVADDR, that was removed on the
 original patch (by mistake, I guess)]
[ehabkost: kill qdev_prop_set_vlan() declaration]
[ehabkost: moved get_fw_dev_path() comment to the original location
 (I don't know why it was moved)]
[ehabkost: removed qdev_exists() declaration]
[ehabkost: keep using 'QemuOpts' instead of 'struct QemuOpts', as
 qdev-core.h includes qemu-option.h]

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>