build: qemu-ga: add 'qemu-ga' build target for w32

Currently POSIX builds rely on 'qemu-ga' target to do qga-only
distributable build. On w32, as with most standalone binary targets,
we rely on 'qemu-ga.exe' target.

Unlike with POSIX, qemu-ga for w32 has a number of related targets
such as VSS DLL and MSI package. We can do the full distributable
qga-only build on w32 with:

  make qemu-ga.exe

or:

  make msi

To make that work, we tie VSS dependencies onto qemu-ga.exe.
However, in reality the DLL isn't part of the binary, so we use a
filter to pull them out of the LINK recipe, which attempts to link
against prereqs for binary targets. Additionally, it could be argued
that VSS is a separate distributable, and shouldn't be implied by
qemu-ga.exe binary target.

To avoid this, we can tie the VSS dependencies only to the 'msi'
target, but that would make it impossible to do a qga-only build of
the w32 distributable without building the 'msi' package, which was
supported in the past.

An alternative approach is to add a new target to build the whole
distributable. w32 allows us to use the same build target we use
on POSIX, 'qemu-ga', since the current binary-only target on w32
is 'qemu-ga.exe'.

To further simplify the build, we also make 'qemu-ga' build the MSI
package if the appropriate ./configure options are set, making the
full qga-only build the same on both POSIX and w32: `make qemu-ga`

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>