guest_os_booting: Fix the nonexist_template case error

[Yingshun]

1. nvram file should be cleared before the testing
2. error message changed

Before the fix:

 (1/1) type_specific.io-github-autotest-libvirt.guest_os_booting.ovmf_nvram.negative_test.nonexist_template: FAIL: Expect should fail with one of ['error: Failed to open file.+: No such file or directory'], but failed with:\n\n\nerror: Failed to start domain 'avocado-vt-vm1'\nerror: Operation not supported: conversion of the nvram template to another target format is not ... (10.01 s)
RESULTS    : PASS 0 | ERROR 0 | FAIL 1 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/log/avocado/job-results/job-2024-12-27T04.16-865d082/results.html
JOB TIME   : 11.99 s

Test summary:
1-type_specific.io-github-autotest-libvirt.guest_os_booting.ovmf_nvram.negative_test.nonexist_template: FAIL
(.libvirt-ci-venv-ci-runtest-d2E5ZF) [root@dell-per750-61 ~]

After the fix:

 (1/1) type_specific.io-github-autotest-libvirt.guest_os_booting.ovmf_nvram.negative_test.nonexist_template: STARTED
 (1/1) type_specific.io-github-autotest-libvirt.guest_os_booting.ovmf_nvram.negative_test.nonexist_template: PASS (9.66 s)
RESULTS    : PASS 1 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/log/avocado/job-results/job-2024-12-27T04.17-2f1317b/results.html
JOB TIME   : 11.75 s
(.libvirt-ci-venv-ci-runtest-d2E5ZF) [root@dell-per750-61 ~]# 

aarch64:

 (1/1) type_specific.io-github-autotest-libvirt.guest_os_booting.ovmf_nvram.negative_test.nonexist_template: PASS (5.33 s)
RESULTS    : PASS 1 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/log/avocado/job-results/job-2024-12-27T04.15-bf87ed2/results.html
JOB TIME   : 9.50 s
(.libvirt-ci-venv-ci-runtest-N3wQe0) [root@ampere-mtsnow-altramax

[Yingshun]

@zhentang-tz Can you plz review this pr? Thanks!

[zhentang-tz]

LGTM