make.rule: fix $(obj) to a real relative path

Makefile.target includes rule.mak and unnested common-obj-y, then prefix
them with '../', this will ignore object specific QEMU_CFLAGS in subdir
Makefile.objs:

    $(obj)/curl.o: QEMU_CFLAGS += $(CURL_CFLAGS)

Because $(obj) here is './block', instead of '../block'. This doesn't
hurt compiling because we basically build all .o from top Makefile,
before entering Makefile.target, but it will affact arriving per-object
libs support.

The starting point of $(obj) is passed in as argument of unnest-vars, as
well as nested variables, so that different Makefiles can pass in a
right value.

Signed-off-by: Fam Zheng <famz@redhat.com>
rule.mak: allow per object cflags and libs

Adds extract-libs in LINK to expand any "per object libs", the syntax to define
such a libs options is like:

        foo.o-libs := $(CURL_LIBS)

in block/Makefile.objs.

Similarly,

        foo.o-cflags := $(FOO_CFLAGS)

is also supported.

"foo.o" must be listed a nested var (e.g. common-obj-y) to make the
option variables effective.

Signed-off-by: Fam Zheng <famz@redhat.com>
build-sys: introduce common-obj-m and block-obj-m for DSO

Add necessary rules and flags for shared object generation.
$(common-obj-m) will include $(block-obj-m), like $(common-obj-y) does
for $(block-obj-y). The new rules introduced here are:

0) For all %.so compiling:

    QEMU_CFLAGS += -fPIC

1) %.o in $(common-obj-m) is compiled to %.o, then linked to %.so.

2) %.mo in $(common-obj-m) is the placeholder for %.so for pattern
matching in Makefile. It's linked to "-shared" with all its dependencies
(multiple *.o) as input. Which means the list of depended objects must
be specified in each sub-Makefile.objs:

    foo.mo-objs := bar.o baz.o qux.o

in the same style with foo.o-cflags and foo.o-libs. The objects here
will be prefixed with "$(obj)/" if it's a subdirectory Makefile.objs.

Also introduce --enable-modules in configure, the option will enable
support of shared object build. Otherwise objects are static linked to
executables.

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

together with object specific flags and libs change of:

    qcow
    linux-aio

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>

build: Add object specific link flags.

qcow.o and linux-aio.o has their own library dependencies for building,
add them to keep ld happy.

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