在我的CentOS 7.2中，我安装了以下软件包：

代码语言：javascript复制yum install -y epel-release qemu-kvm libvirt virt-install bridge-utils然后当我向ip a展示时：

代码语言：javascript复制[root@localhost ~]# ip a

1: lo: mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000

link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00

inet 127.0.0.1/8 scope host lo

valid_lft forever preferred_lft forever

inet6 ::1/128 scope host

valid_lft forever preferred_lft forever

2: em1: mtu 1500 qdisc mq state UP group default qlen 1000

link/ether e8:9a:8f:50:4a:dc brd ff:ff:ff:ff:ff:ff

inet 13.19.241.25/29 brd 13.19.241.31 scope global em1

valid_lft forever preferred_lft forever

inet 13.19.241.26/8 brd 13.255.255.255 scope global em1

valid_lft forever preferred_lft forever

inet6 fe80::ea9a:8fff:fe50:4adc/64 scope link

valid_lft forever preferred_lft forever

3: em2: mtu 1500 qdisc mq state UP group default qlen 1000

link/ether e8:9a:8f:50:4a:dd brd ff:ff:ff:ff:ff:ff

inet 103.193.241.25/29 brd 103.193.241.31 scope global em2

valid_lft forever preferred_lft forever

inet6 fe80::ea9a:8fff:fe50:4add/64 scope link

valid_lft forever preferred_lft forever

4: virbr0: mtu 1500 qdisc noqueue state DOWN group default qlen 1000

link/ether 52:54:00:f9:a9:cf brd ff:ff:ff:ff:ff:ff

inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0

valid_lft forever preferred_lft forever

5: virbr0-nic: mtu 1500 qdisc pfifo_fast master virbr0 state DOWN group default qlen 1000

link/ether 52:54:00:f9:a9:cf brd ff:ff:ff:ff:ff:ff您可以看到添加了两个网络设备：virbr0和virbr0-nic。他们是什么，他们有什么作用？

安装这两个设备的软件包是哪个？