module: implement module loading

This patch adds loading, stamp checking and initialization of modules.

The init function of dynamic module is no longer directly called as
__attribute__((constructor)) in static linked version, it is called
only after passed the checking of presense of stamp symbol:

    qemu_stamp_$(date +%s$$$RANDOM)

With this, modules built from a different tree/version/configure will
not be loaded.

The module loading code requires gmodule-2.0.

Signed-off-by: Fam Zheng <famz@redhat.com>
Makefile: install modules with "make install"

Install all the modules to ${MODDIR}.

Signed-off-by: Fam Zheng <famz@redhat.com>
.gitignore: ignore module related files (dll, so, mo)

Signed-off-by: Fam Zheng <famz@redhat.com>
block: convert block drivers linked with libs to modules

The converted block drivers are:

    curl
    iscsi
    rbd
    ssh
    glusterfs

no longer adds flags and libs for them to global variables, instead
create config-host.mak variables like FOO_CFLAGS and FOO_LIBS, which is
used as per object cflags and libs.

Signed-off-by: Fam Zheng <famz@redhat.com>