qemu: avoid maybe-uninitialized warning by GCC 10

qemu: avoid maybe-uninitialized warning by GCC 10

GCC 10 complains about "well_formed_uri" may be used uninitialzed.
Even though it is a false positive, we can easily avoid it.

Avoiding
  ../src/qemu/qemu_migration.c: In function ‘qemuMigrationDstPrepareDirect’:
  ../src/qemu/qemu_migration.c:2920:16: error: ‘well_formed_uri’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
    2920 |             if (well_formed_uri) {
         |                ^

Signed-off-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>