block: Use GString instead of QString to build filenames

QString supports modifying its string, but it's quite limited: you can
only append.  Just one caller remains:
bdrv_parse_filename_strip_prefix() uses it just for building an
initial string.

Change it to do build the initial string with GString.  This is
another step towards making QString immutable.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201211171152.146877-20-armbru@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>