In commit 76b9aba2 I refactored how the function treats the readonly
flag which introduced a bug when we'd not allow to force read-write
state for an image.

This created problems with blockjobs where we need to temporarily
force images to have read-write permissions.

Rename QEMU_DOMAIN_STORAGE_SOURCE_ACCESS_READ_ONLY to
QEMU_DOMAIN_STORAGE_SOURCE_ACCESS_FORCE_READ_ONLY and also introduce
a complement QEMU_DOMAIN_STORAGE_SOURCE_ACCESS_FORCE_READ_WRITE which
will allow to force write access.

https://bugzilla.redhat.com/show_bug.cgi?id=1717768Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>