qemu: Refactor variables for extracting flags in qemuDomainBlockCopyCommon

qemu: Refactor variables for extracting flags in qemuDomainBlockCopyCommon

Add separate booleans for extracting VIR_DOMAIN_BLOCK_COPY_REUSE_EXT and
VIR_DOMAIN_BLOCK_COPY_SHALLOW from '@flags' and also change 'reuse' into
'existing'.

qemuMonitorDriveMirror requires the unmodified state of the flags to
pass to qemu and also we use the value a few times internally. Extract
it separately now.

The 'reuse' flag did not indicate reusing of the file as much as the
fact that the storage is existing and thus should not be created, so
modify the name to reflect this.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>