build: fix VPATH builds

build: fix VPATH builds

After the recent libvirt-qemu library addition, VPATH builds fail with:

  CC     libvirt_qemu_la-libvirt-qemu.lo
In file included from ../../src/libvirt-qemu.c:29:
../../include/libvirt/libvirt-qemu.h:17:22: error: libvirt.h: No such file or directory
...
  CCLD   libvirt-qmeu.la
/usr/bin/ld: cannot open linker script file libvirt_qemu.syms: No such file or directory

This fixes both issues (there are still some documentation VPATH issues,
but those don't show up with 'make check').

* configure.ac (LIBVIRT_QEMU_SYMBOL_FILE): While libvirt.syms is
generated and lives in $(builddir), libvirt_qemu.syms is static
and lives in $(srcdir).
* include/libvirt/libvirt-qemu.h (includes): Pull in libvirt.h via
the public location, since this is a public header.