update_vm: check whether opt is set before deleting

update_vm: check whether opt is set before deleting

otherwise we get lots uninitialized warnings:

update VM 600: -delete unused7
Use of uninitialized value $data in split at /usr/share/perl5/PVE/JSONSchema.pm line 533.
Use of uninitialized value in concatenation (.) or string at /usr/share/perl5/PVE/API2/Qemu.pm line 1012.
Use of uninitialized value $volid in pattern match (m//) at /usr/share/perl5/PVE/QemuServer.pm line 1824.
Use of uninitialized value $volid in pattern match (m//) at /usr/share/perl5/PVE/Storage/Plugin.pm line 201.
Use of uninitialized value $volid in concatenation (.) or string at /usr/share/perl5/PVE/Storage/Plugin.pm line 205.

vs:

update VM 600: -delete unused7
cannot delete 'unused7' - not set in current configuration!

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>