qga: ignore EBUSY when freezing a filesystem

qga: ignore EBUSY when freezing a filesystem

the current implementation fails if we try to freeze an
already frozen filesystem. This can happen if a filesystem
is mounted more than once (e.g. with a bind mount).

Suggested-by: Christian Theune <ct@flyingcircus.io>
Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
(cherry picked from commit ce2eb6c4a044d809caf4dc4e08aed77678f9760e)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>