Allow migration over IPv6 by listening on [::] instead of 0.0.0.0
when QEMU supports it (QEMU_CAPS_IPV6_MIGRATION) and there is
at least one v6 address configured on the system.

Use virURIParse in qemuMigrationPrepareDirect to allow parsing
IPv6 addresses, which would cause an 'incorrect :port' error
message before.

Move setting of migrateFrom from qemuMigrationPrepare{Direct,Tunnel}
after domain XML parsing, since we need the QEMU binary path from it
to get its capabilities.

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=846013