qemu: backup: Enable full backup support

qemu: backup: Enable full backup support

qemuBackupBegin can take a full backup of the disks (excluding any
operations with bitmaps) without the need to wait for the
blockdev-reopen support in qemu.

Add a check that no checkpoint creation is required and the disk backup
mode isn't VIR_DOMAIN_BACKUP_DISK_BACKUP_MODE_INCREMENTAL.

Call to virDomainBackupAlignDisks is moved earlier as it initializes the
disk backup mode if not present in user config.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>